The invention relates generally to motor control and more particularly, to open-loop step motor control systems that reduce acoustic noise while maintaining sufficient torque.
A step motor applies torque to its load in a series of discrete steps and consequently may act as a sound transducer, generating an audible tone with a fundamental frequency equal to its step rate. If the motor is to be operable over a wide range of step rates, one or more of these rates will probably excite resonant frequencies of the motor's mechanical load, or of the motor itself, resulting in the production of objectionable amounts of acoustic noise and in less efficient operation.
In the medical equipment field, it is usually desirable to lower the noise level of the equipment for the benefit of the patient and others. For example, infusion pumps containing step motors are generally located next to a patient and may operate for hours. It can be disturbing to a patient when the pump generates a large amount of noise. Additionally, certain medical equipment, including many infusion pumps, must be powered by a portable power supply having a limited reservoir of power, such as batteries, and therefore the equipment must be designed to consume as little power as possible. In this way, the equipment can support the patient for as long as possible before a battery change or recharge is required. Thus, lowered levels of noise and lowered levels of power consumption are desirable characteristics in infusion pumps and other medical equipment.
A source of acoustic noise in a step motor is the wave shape of the motor drive. The simplest means of driving a step motor is the "full step" mode in which a two-phase motor is driven by a current or voltage square wave of constant magnitude. In this mode, each step corresponds to one of 2.sup.N possible motor winding current polarity states where N is the number of motor windings (or phases). This type of drive generates acoustic noise with high harmonic content due to the high angular acceleration resulting from the high rate of change of torque that occurs at the leading edge of each step. Additionally, where the drive rate is sub-optimum and the rotor reaches its position before the winding currents are switched, a damped oscillation of the rotor about the motor magnetic field position may occur with resulting excess noise and wasted power in providing negative torque to hold the rotor and energy is lost in merely heating the windings due to the resistance encountered.
The noise component can be reduced if the magnitude of the torque pulses is decreased by reducing the magnitude of the motor drive pulse. Such a reduction, however, also reduces the motor's available torque reserve, resulting in an increased risk of motor stall or "pull out" where "pull out" refers to the loss of synchronization because the load on the motor exceeds the power available to the motor to move the load, thus the motor "pulls out" of its movement cycle and loses one or more steps. This condition can result in positioning errors due to the lost steps.
Having an adequate torque reserve is necessary in the case where certain undesirable conditions may occur. In the medical field where a step motor is used to drive a pumping mechanism, such as a peristaltic pump, the head heights of the infusion fluid change, infusates may be particularly viscous, and cold temperatures may require greater power to move the peristaltic mechanism, for example. The motor's rated torque should be high enough to handle all of these circumstances but in any case, its rated torque plus its torque reserve must be high enough or motor pullout may occur. Typically, a mechanism has a rated torque and a torque reserve. In one embodiment, the reserve torque is set at seventy percent of the rated "no stall" torque.
It has been found that motor noise can be significantly reduced by the technique known as "microstepping." "Microstepping" is a means of driving a motor through a step with a series of current magnitude states that generate smaller angular displacements of the motor magnetic field vector position. The sum of these displacements equals that of one step. Because instantaneous torque is approximately a sinusoidal function of the angular displacement of a motor's field vector position from its rotor position, a smaller angular displacement results in a lower instantaneous torque. A lower instantaneous torque generates an angular acceleration at the leading edge of each "microstep" smaller than that which would be generated at the leading edge of each step in "full step" drive mode. The effect is to spread the large acceleration that normally occurs at the beginning of a step over the entire step as a series of small accelerations, thus reducing the level of acoustic noise.
However, "microstepping" is not a satisfactory noise reduction technique if power consumption must be limited, as in battery-powered applications. In the microstep technique, motor winding currents, that define the state sequence, must be maintained throughout the sequence, resulting in relatively high power consumption. Other lower power consumption step modes are available, such as "one phase on" mode where the winding currents are turned off after the initial acceleration to conserve power. However, these modes are noisier than the microstepping mode. Microstepping is also not desirable where controller bandwidth is limited. As the number of microsteps increases, the controller bandwidth requirement increases requiring greater hardware capability to support a faster clock speed. This greater ability results in increased expense and complexity.
The type of motor drive circuit can also have a direct effect on expense. For example, closed-loop drive circuits typically require sensors to provide the necessary feedback for control. The cost of the sensors as well as the additional processor bandwidth required to use the sensor inputs to control the drive circuit can result in a substantial increase in cost. An open-loop control system is preferable in this regard.
Thus, greater control over power consumption is important in applications where long battery life is desired. Providing excessive power to the step motor windings can cause wasted power and shortened battery life. Power can be lost as heat due to winding resistance. Similarly, moving the motor at its resonance frequency is inefficient and can result in wasted power because relatively little torque is created from the large input power that is provided to the motor. Thus precise motor control is desirable to avoid wasting limited energy.
Hence those skilled in the art have recognized the need for lowering the acoustic output of medical devices while also lowering the power consumption, but retaining an adequate torque reserve. Additionally, those skilled in the art have also recognized the need for an open-loop control system to reduce hardware and processor costs. The present invention fulfills these needs and others.